The present invention relates, among other aspects, to a method of operating an electrochemical cell to produce a biocidal solution and apparatus for producing a biocidal solution by way of the electrolytic treatment of an aqueous chloride solution.
In hospitals it is important to provide appropriate levels of sterility, particularly in operating theatres and other situations where invasive treatments are performed. Surgical instruments and other apparatus must be sterilised or disinfected, depending on their application, before use in order to reduce the risk of bacterial infection. One method of sterilisation is the application of heat and pressure in an autoclave. However, this is not suitable for some medical apparatus, such as heat-sensitive endoscopes.
A typical method employed for reprocessing heat sensitive instruments involves the use of chemical biocides, such as glutaraldehyde. This can be unsatisfactory due to improper or incomplete disinfection. Furthermore, exposure to glutaraldehyde fumes can cause asthma and dermatitis in healthcare staff. Also, glutaraldehyde is believed to have relatively low sporicidal activity. Moreover, other disinfectants, such as chlorine dioxide and peracetic acid may suffer from similar handling problems as glutaraldehyde.
For some years, it has been known that electrochemical activation of brine produces a super-oxidised water which is suitable for many applications including general disinfection in medical and veterinary applications and the sterilisation of heat-sensitive endoscopes. There has been a recent interest in the use of super-oxidised water as a disinfectant because of its rapid and highly biocidal activity against a wide range of bacteria, fungi, viruses and spores. Also, super-oxidised water is an extremely effective sterilising cold non-toxic solution which is free from highly toxic chemicals, thereby presenting reduced handling risk.
GB 2253860 describes the electrochemical treatment of water through an electrolytic cell. Co-axially arranged cylindrical and rod electrodes provide anode and cathode (working and auxiliary) flow chambers which are separated by a porous membrane made of a ceramic based on zirconium oxide.
Water is fed from the bottom to the top of the device through the working chamber. Simultaneously, water having a higher mineral content flows through the auxiliary chamber to a gas-separating chamber. An electric current is passed between the cathode and anode through the water in both chambers and the porous membrane separating the chambers. Water flowing through the auxiliary chamber recirculates to the auxiliary chamber by convection and by the shearing forces applied to the water through the rise of bubbles of gas which are generated on the electrode in the auxiliary chamber. The pressure in the working chamber is higher than that in the auxiliary chamber, and gaseous electrolysis products are vented from the gas-separating chamber by way of a gas-relief valve. A change of working mode from cathodic to anodic water treatment is achieved by changing polarity.
This electrolytic process acts on salts and minerals dissolved in the water, such as metal chlorides, sulphates, carbonates and hydrocarbonates. Where the working chamber includes the cathode, the alkalinity of the water may be increased through the generation of highly soluble metal hydroxides. Alternatively, the electrolytic cell may be switched so that the working chamber includes the anode, in which case the acidity of the water is increased through the generation of a number of stable and unstable acids.
A similar electrolytic cell is described in GB 2274113. This cell includes two coaxial electrodes, separated by an ultra-filtration diaphragm (porous membrane) based on zirconium oxide, thereby defining a pair of coaxial chambers. A current source is connected to the electrodes of a plurality of cells via a switching unit to enable polarity alteration of the electrodes to eliminate deposits on the cathode and to connect the cells electrically either in series or parallel.
WO 98/13304 describes the use of such an electrolytic cell in an apparatus to process a liquid, such as water. A liquid is supplied to the cathode chamber only and part of the output from the cathode (catholyte) is recycled to the input of the anode chamber. This input serves as the total supply to the anode chamber. In situations where not all of the solution output from the cathode chamber is recycled to the input of the anode chamber, a proportion of the output from the cathode chamber is drained to waste, this proportion being measured by a flow meter. A constant-voltage DC supply is applied between the anode and the cathode, and the pH and redox potential of the treated solutions are measured and maintained by controlling flow rates through the cell.
A method and apparatus for producing a sterilising solution is described in GB 2316090, the subject matter of which is incorporated herein by reference, wherein a supply of softened water is generated by passing water through an ion-exchange water softener. A saturated salt solution, generated by mixing softened water with salt, is passed through an electrolytic cell to produce a sterilising solution, or used to regenerate the ion-exchange resin in the water softener.
However, all of the systems described above have drawbacks and difficulties. For example, the variable factors, such as the degree of electrolysis in the electrolytic cells, the concentration of dissolved salts and minerals and the flow rates, the fluctuations in electricity supply, ambient temperature and the variability of incoming water supplies present a barrier to ensuring a consistent supply of sterilising or, more correctly, biocidal solution. Thus in order to ensure delivery of a biocidal solution, the electrochemical systems described all rely upon expert intervention to calibrate the cells at the time of installation and to re-calibrate whenever the chemistry of the water supply changes to any significant degree.
As an illustration, the pH of the solution output from the anode chamber (anolyte) may be regulated by adjusting the flow rate of catholyte drained from the cathode chamber. This results in changes to the anolyte flow rate and consequently in changes to the electrochemistry taking place in the electrolytic cell.
Also, the performance of all the above cells and methods is highly dependent on the alkalinity of the water and aqueous salt solutions being treated. In Europe, for example, the alkalinity of potable water can vary from very low (3-15 ppm CO3 as CaCO3) to very high (470 ppm CO3) from one geographical region to another. This means that a cell which is calibrated to produce a biocidal solution of given composition in a first geographical location may not produce the same biocidal solution in a second location, making re-calibration necessary. This is a time-consuming and laborious task.
Minimising variation is important to ensure a supply of solution having the required properties, e.g. biocidal activity and pH, especially when thorough sterilisation is required to maintain the health of a population.
Furthermore, it is important to be able to control to a fine degree the final composition of any biocidal solution produced, since the solution must have a high enough concentration of, say, available free chlorine (AFC) to be sufficiently biocidal, but not so high as to corrode or otherwise damage any equipment which is being sterilised. A still further disadvantage of the apparatus described in the prior art is that they are prone to a high level of wastage. Up to half of the initial supply of aqueous salt solution may be discarded after being passed through the cathode chamber. This is especially pertinent where resources such as water are limited or costly.
In the Applicant's experience, none of the above systems is suited to providing a wholly reliable or autonomous supply of biocidal solution. As will be readily appreciated, a “sterilising” solution which does not meet the required level of biocidal efficacy carries a risk of allowing an instrument to spread infection. Moreover, the end user will not be able to detect by visual inspection alone whether the biocidal solution from any one of these systems is within or outside specification.